Commas should not be used
a. to convey contrast.
b. after yes or no in answering a question.
c. to explain a specific statement.
d. following interjections.

A comma may be used correctly to do any of the following:
(a) join sentence when used with a conjunction. 
(b). set off an introductory word, clause, or phrase. 
(c) set off interrupting words or expression

Examples:

When he came, his mother ran unto him.
Talking about bugs, I don't like any of them.
Hey, did you do the thing I told you to do?
if you don't know how to answer that, you can ask me for help.